Though, at first, collisions by aircraft while flying in clear skies would not seem to be significant, the contrary is often actually the real situation. Hazards exist even though there may not be bad weather or other obvious problems. Clear weather may lull pilots into believing that they can relax at such times. Thus, for instance, in order to avoid other airplanes it is necessary to constantly survey what can be seen in the entire frontal area above, on both sides, below and to the rear of the aircraft. However, due to aircraft structures such as wings and aft lower fuselage, large volumes of airspace are blocked from a pilot's view. Also, the cockpit workload of the pilot which includes checking charts, radio settings and gauge readings can draw attention and interfere with a pilot's normal visual scanning practice. Many mid-air collisions are a result of these problems.
Ground based radar can help but this is not available in many areas and heavy controller workload may cause pilots to go unnotified of potential collision hazards. Even in airspace where radar coverage can be provided, not all aircraft are in contact with the controller or capable of telling their altitudes. Thus, the controller may have no idea two aircraft are on a collision course.